Protein Folding

The Molten Globule State

Summary

The molten globule is a compact folding intermediate with native-like secondary structure but a fluid, dynamic tertiary structure lacking a fixed hydrophobic core. It represents a key waypoint on the protein folding pathway between the unfolded state and the native structure.

Key Points

  • 1Native-like secondary structure but fluid tertiary structure
  • 2Compact state with exposed hydrophobic surfaces (ANS binding)
  • 3Key intermediate in the protein folding pathway
  • 4Important for membrane translocation and chaperone recognition

The molten globule state bridges the gap between unfolded and native proteins, providing critical insights into the protein folding process and energy landscape.

Defining Characteristics

The molten globule is characterized by several distinctive features:

Secondary Structure

- Native-like secondary structure content (α-helices, β-sheets)

  • Detectable by circular dichroism (CD) spectroscopy
  • Similar secondary structure topology to the native state

    • Tertiary Structure

    - Fluid and dynamic side chain packing

  • No fixed hydrophobic core
  • Rapid interconversion between conformations
  • ANS (8-anilino-1-naphthalenesulfonate) binding indicates exposed hydrophobic surface

    • Compactness

  • More compact than the unfolded state
  • Slightly expanded (~10-30%) compared to native state
  • Radius of gyration intermediate between unfolded and folded

    • Detection and Characterization

    Experimental Signatures

    - CD spectroscopy: Near-native far-UV signal (secondary structure), reduced near-UV signal (tertiary structure)

    - ANS fluorescence: Enhanced binding due to exposed hydrophobic surfaces

    - NMR: Broad peaks indicating conformational exchange

    - Hydrogen exchange: Intermediate protection compared to native state

    Conditions Favoring Molten Globules

    - Low pH (acid-induced molten globule)

    - Moderate denaturant concentrations

    - Removal of cofactors

    - Kinetic intermediates during folding

    Role in Protein Folding

    The Folding Funnel Perspective

    In the energy landscape model:

  • Unfolded state at the top of the funnel

    • 2. Molten globule represents a broad energy well partway down

  • Native state at the narrow bottom

    • Kinetic Importance

    - Acts as an obligatory intermediate in many folding pathways

    - Hydrophobic collapse occurs first, forming molten globule

  • Secondary structure is then stabilized
  • Final tertiary contacts lock in native structure

    • The Framework Model

    Some proteins follow a "framework" mechanism:

  • Secondary structure forms in the molten globule
  • Acts as a scaffold for tertiary structure assembly
  • Alternative to the nucleation-condensation model

    • Biological Significance

    Translocation and Import

  • Proteins must unfold to cross membranes
  • Molten globule-like states facilitate:

    • - Mitochondrial protein import

    - ER translocation

    - Secretion through bacterial membranes

    Chaperone Interactions

  • Chaperones often recognize molten globule states
  • Hsp70 binds exposed hydrophobic regions
  • Prevents aggregation during the vulnerable intermediate stage

    • Disease Implications

  • Molten globules can be aggregation-prone
  • Transient exposure of hydrophobic surfaces risks misfolding
  • Some amyloidogenic proteins populate molten globule states

    • Equilibrium vs Kinetic Molten Globules

    Equilibrium Molten Globules

  • Stable under specific conditions (pH, denaturant)
  • Can be studied at leisure
  • Examples: α-lactalbumin at pH 2, apo-myoglobin

    • Kinetic Molten Globules

  • Transient intermediates during folding
  • Detected by stopped-flow and rapid mixing
  • Typically millisecond to second timescales
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